Method for coating the surface of a thin wire with a layer of another metal

ABSTRACT

A method for manufacturing a dumet wire or a similar thin metal wire coated with a different metal wherein a small capillary device, which provides above the level of a molten metal bath a small capillary space to be filled with the molten metal by virtue of capillary attraction, is dipped into the molten metal bath, and a metal wire to be coated is passed through a molten metal pool created in the capillary space.

I Umted States Patent 1191 11 11 3,765,930 Miyano Oct. 16, 1973 [54] METHOD FOR COATING THE SURFACE 1,033,912 7/1912 Lendi 118/405 X OF A THIN WIRE WITH A LAYER 01? 1,076,526 10/1913 Shiverick 118/401 X 1,454,224 5/1923 Schmidt 118/405 UX ANOTHER METAL 2,934,458 4/1960 Budd et al..... 117/114 R x [75] Inventor: Tadashi Miyano, Kanagawa, Japan 3,196,830 7/1965 Lehovec 118/401 2,255,436 9/1941 Olson 118/405 x Asslgneel Tokyo shlbaul'a Electl'lc (30-, -1 2,914,419 11/1959 Oganowski 117/114 A x Kawasaki-shi, Japan [22] F1 d J l 6 1971 FOREIGN PATENTS OR APPLICATIONS u y 1,192,213 5/1970 Great Britain 117/114 B [21] Appl. No; 160,096

1 Primary Examiner-Ralph S. Kendall [30] Foreign Application Priority Data y Flynn et .Iuly 10, 1970 Japan 45/59850 [57] ABSTRACT [52] U.S.C1. 117/114 R, 117/114 C, 117/115,

l 18 M01 11 8/405 A method for manufacturmg a dumet wire or a s1m1lar [51] Int. Cl (323C U08 C23: 1H0 thin metal wire coated with a different metal wherein [58] Field of 2 R 114 A a small capillary device, which provides above the 1 8/401 level of a molten metal bath a small capillary space to be filled with the molten metal by virtue of capillary [56] References Cited attraction, is dipped into the molten metal bath, and a metal wire to be coated is passed through a molten UNITED STATES PATENTS metal pool created in the capillary space. 2,937,108 5/1960 Toye 117/119 X 2,325,129 7/1943 Hardy 118/401 8 Claims, 6 Drawing Figures METHOD FOR COATING THE SURFACE OF A THIN WIRE WITH A LAYER OF ANOTHER METAL BACKGROUND OF THE INVENTION This invention relates to an improved method for coating the surface of a thin metal wire with a layer of another metal.

The dumet wire is so far manufactured according to the following method: the surface of a wire of a ferronickel alloy is covered with a brass ribbon and the thus prepared composite piece is compactly inserted into a copper tube. The thus prepared composite article is cold-drawn in one direction and the drawn piece is then hot-drawn in the reverse direction. In this method, however, if the temperature is in excess of 900C when the composite article is preheated prior to the hot drawing, the brass part of the article may melt in places. When the hot drawing is carried out in this condition, the brass is fixed to the core wire at those places, which create greater resistance to the drawing and the smooth and uniform elongation is impaired at those places, and thus damage of the copper layer often results.

By way of overcoming the above-mentioned difficulty, the following method is known. That is, a wire of a ferro-nickel alloy is tightly inserted into a copper tube directly without a brass ribbon being used, and the composite article is cold-drawn until the cross sectional contraction reaches 12 to 16 percent. The thus colddrawn article is then hot-drawn at a temperature of 850900C in a neutral or reducing atmosphere unitl the same contraction is reached. The drawn composite wire is cooled once, and it is subjected to another cycle of cold drawing and hot drawing. Finally the wire is again subjected to the same cycle in the reverse direction. In this process there is no fear of damage of the copper layer. But this process is disadvantageous in that the steps are extremely complicated and the surface of the coated wire may be stained with lubricating oil, etc.

SUMMARY OF THE INVENTION An object of this invention is to manufacture a thin wire coated with a layer of another metal, such as the dumet wire or grid wire, which is in no respect inferior to the prior art product, by a very simple process.

Another object of this invention is to manufacture a thin wire coated with a layer of another metal which has a finer and smoother surface.

These objects may be attained in accordance with the present invention by dipping a small capillary device into a molten metal bath, said device providing a small capillary space to be filled with the molten metal by virtue of capillary attraction, and passing a metal wire through the small molten metal pool held in the capillary device above the level of the molten metal bath.

Further objects and advantages of the present invention will become apparent, and this invention will be better understood by perusal of the following description, reference being made to the accompanying drawmg.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partly perspective longitudinal sectional view of an apparatus used for the present invention, wherein a small capillary device holding therein a small molten metal pool above the level of the molten metal bath, through which a thin wire is passed, is shown;

FIGS. 2 to 5 exemplarily show varied forms of the above-mentioned small capillary device in perspective view; and

FIG. 6 shows another embodiment of this invention, wherein the wire is vertically passed through the molten metal.

DETAILED DESCRIPTION OF THE INVENTION When a small capillary device made of a high melting metal such as molybdenum or tungsten comprising a pair of wall plates adjacently facing each other with a small space or gap therebetween is dipped into a molten bath of copper or gold, the molten metal is spontaneously drawn up in the small space or gap by virtue of capillary attraction. This is a surprising finding made by the present inventor.

When a thin wire at room temperaure is continuously passed through the molten metal held in the small capillary space above the molten metal level, the wire is not only coated smoothly and uniformly with the metal but the product is free from staining of the surface with dross. This is a further surprising discovery by the present inventor.

The strength of said capillary attraction varies depending upon the correlation between species and temperature of molten metal, material of the capillary device and dimensions thereof. It has been discovered: The height of a capillary column is the greatest, when a capillary device made of metallic molybdenum is dipped into molten gold, and said height can be as high as 50 mm when the distance of a capillary gap or the diameter of a capillary tube is 2 mm. The second greatest height is attained when tungsten is used for the device. Other metals having high melting point can be successfully used. It is needless to say that these metals must not alloy with molten metals. when in contact therewith.

The above-mentioned capillary phenomenon takes place when the temperature of the bath is higher than about 1 C for copper and higher than about l080C for gold.

The shape of the capillary device can be widely varied. Initially the inventor used a capillary device of groove type as shown in FIG. 1 as member 6. The distance between the two walls composing a capillary space is 2 mm at the greatest, and molten metal comes up in the gap so as to provide a molten metal pool 9 when the bottom of the device is put under the surface of the molten metal.

As mentioned above, various shapes of the capillary device are possible, some examples of which are shown in FIGS. 2 through 5. FIG. 2 shows a device in which the middle part of the walls are-bulged so that a larger amount of molten metal can be held therein. This device is suitably used with the bottom of the device upside and the open end immersed in the bath. The distance between the walls is 2 mm at the greatest and the diameter of the bulged part is 5 mm at the greatest.

FIG. 3 shows a device the walls of which are not in parallel but are wider apart toward the open end. This device is used with the bottom upside. The distance be tween the walls is a maximum of 2 mm at the closed off end and a maximum of 5 mm at the lower open end.

FIG. 4 shows a device which is substantially of the same structure as the device in FIG. 1, except that the walls and bottom are far thicker. This device is used with the bottom sideward. There is no specific meaning in that the walls are made thick, but this merely teaches that such design is possible.

FIG. represents a tube device. A simple capillary tube, the inner diameter thereof being up to 2 mm does not provide a space for the molten metal pool sufficient enough for coating, and so the middle of the capillary tube is bulged up to 5 mm in diameter.

In all the capillary devices as shown in these figures, holes 8 through which a thin wire to be coated runs are provided in the side walls. When a wire is horizontally passed through the holes of a capillary device, the one end of which is immersed in a molten metal bath, the surface of the wire is uniformly coated with the molten metal, since the wire runs through the molten metal pool. When the devices of groove type as shown in FIGS. 1, 2 and 3 are used, the wire can be passed through the capillary gap between the walls in parallel therewith. In an extreme case, a mere pair of metal plates arranged in parallel with a distance up to 2 mm which are substantially vertically placed so that the lower ends thereof are immersed in a molten metal bath, also function as a capillary device, though not shown in the drawing.

The device represented by FIG. 5 is limited in the space for the molten metal pool, and so processing of a thicker wire becomes difficult. If this type of device is to be used for processing of such thicker wire, the bulge of the middle portion thereof should be considerably great. However, this makes raise of the molten metal difficult because of decrease in capillary attraction. This difficulty can be overcome by connecting the upper end of the device to a vacuum system and thus maintaining the head-space of the molten metal pool in a reduced pressure, resulting in that said reduced pressure helps the drawing up of the molten metal.

In the above explained embodiment of this invention, the wire runs horizontally. However, the wire can be passed in a substantially vertical direction, if it is convenient or desirable. This can be performed by using a little complicated apparatus as shown in FIG. 6, comprising a special capillary device 6 and a special vessel for molten metal 4. The vessel 4 is furnished with a cylindrical passage 14 for the wire protruded from the bottom thereof, through which the wire runs without contacting the molten metal in the vessel. The capillary device 6 is of a structure like a double-walled cap, which is provided with holes 8 at the center of the head part for the running wire, and is placed over the protruded cylindrical passage 14 of the vessel so that the holes and the passage align, and the open lower edges of the cap are immersed in the molten metal. The distance between the double walls is 2 mm at the greatest. Then the space between the walls or the jacket 9 is filled with the molten metal by capillary attraction. Therefore, if a wire 10 is passed through the passage 14 and the holes 8 and 8, the wire is coated with the metal.

In the method of this invention, through a molten metal, which is maintained at a considerably high temperature, there passes a wire at a far lower temperature. Therefore the metal solidifies almost instantaneously on the surface of the wire, and thus the running wire is continuously coated with the metal layer. In this case, when the time of contact of the wire and the melt is prolonged, the thickness of the coated layer becomes thinner because of temperature rise of the wire. On the contrary, the shorter the contact time, the thicker the coated layer becomes. Further, shorter contact time between the wire and the melt is advantageous in that eccentric and irregular coating does not occur.

There is a significant relation between the running speed of wire and the temperature of molten metal. For instance, when the temperature of a melt is higher than 1300C for copper or 1280C for gold, the thickness of the resulting coating is no more than 1 2 t, which is an impractical thickness, even if the running speed of a wire is increased to 15 m/min or higher.

The relation between temperatures of a molten metal bath, running speeds of a wire and thicknesses of resulting coating observed when a molybdenum wire 20 p. in diameter was coated with gold is shown in Table l. [n this case, th distance of the capillary gap was 2 mm in all runs.

TABLE I Temperature of Running speed of Thickness of coated melted metal bath thin wire metallic layer (C) (mlmin) (1 about 1080 5.5 20 to 30 1110 7 20 to 30 H30 9 20 to 30 H50 11 20 to 30 1200 I2 10 to 20 I280 15 5 to 10 All coated layers obtained under the conditions indicated in Table 1 had smooth and beautiful apperance. However, when the running speed of a wire was increased in excess of the value given in the table for each temperature, the surface of the resulting coating became rough. To the contrary, when the running speed was decreased lower than the indicated value, the coated layer became thinner, and when the speed was extremely lowered, the coated layer took an irregular bead-like appearance.

As the coating metal, copper and gold are most frequently used, but silver, silver solder, aluminum, etc. can be also used. As the thin wire to be coated, wires of molybdenum, tungsten, fcrro-nickel alloys, ironnickel-cobalt alloys, etc. are applicable to this invention.

Now the invention is illustrated by way of working examples.

EXAMPLE I A grid wire covered by a layer of gold was manufactured using an apparatus the outline of which is shown in FIG. 1.

A vessel 4 of high purity zirconia containing a molten gold bath 5 was placed in a tubular furnace 3 equipped with an electric heating element 2 imbedded in the furnace wall I. A groove type capillary device 6 was placed in the molten gold bath so that the bottom thereof was immersed in the bath. The capillary device was fixed at a predetermined position by a means which is not shown in the drawing. The distance of the two parallel walls 7 of the capillary device was 2 mm, and the height of the walls was about 20 mm. The walls were provided with a hole 8 respectively at the position suitable for passing a wire to be coated. Thus the melted gold was raised in the capillary gap to a height above the position of the holes.

A reducing atmosphere was establishd in the furnace 3 by passing hydrogen gas therethrough, and a molybdenum wire 20 p. in diameter was passed through the holes 8 at the speed of 12 m/min. Thus a thin grid wire coated with a p. layer of gold was obtained.

The conventional gold-coated grid wire is prepared by the electroplating method. The grid wire prepared in accordance with this invention is quite free from inclusion of impurities such as cyanates and oxides, and has no pin holes which are incidental to the electroplating. The grid wire prepared in accordance with this invention remarkably improves the characteristics of an electron tube when it is used as a control means of a secondary emission of a grid of any kind.

It is needless to say that the molten metal bath has to be replenished with the metal by a suitable means as the coating operation is continued and the molten metal is consumed.

EXAMPLE 2 Using an apparatus similar to that of Example 1, a dumet wire was continuously manufactured. A core wire of a ferro-nickel (Fe 58 Ni 42 0.45 mm in diameter was used, and a capillary device represented by FIG. 2 was employed. The capillary gap distance thereof was 2 mm, and the inner diameter of the bulged part is 5 mm. This device was partly immersed in a melted copper bath of ll30C in the same way as shown in FIG. 1, and a core wire at a room temperature was continuously passed through the holes in the walls at the speed of 15 m/min. The obtained dumet wire had a coating p in thickness, and the surface was very homogeneous and pure, free from stain which is incidental to a product of the drawing method in which a lubricating oil is used. The cross section showed that there was no offset in adhesion of the coating layer.

What we claim is:

1. A method for coating a thin high melting refractory metal wire selected from the group consisting of molybdenum, tungsten, iron-nickel, and iron-nickelcobalt, with a coating metal selected from the group consisting of gold, copper, silver, silver solder and aluminum comprising imersing a small capillary decice into a molten bath of said coating metal, said molten bath and said capillary device being within a furnace, said capillary device providing a capillary space containing a pool of said molten coating metal and positioned above the level of said molten bath, and continuously passing said thin wire which is at room temperature at a constant speed into said furnace and through said pool of molten coating metal while maintaining a reducing atmosphere in said furnace whereby a thin smooth clean coating of said coating metal is applied on said wire.

2. The method according to claim 1 wherein the small capillary device comprises a pair of walls facing each other and a bottom connecting ends of the two walls.

3. The method according to claim 1 wherein the small capillary device is a short piece of a capillary tube.

4. The method according to claim 2 wherein the small capillary device has holes in the walls through which a wire to be coated is passed.

5. The method according to claim 3 wherein the capillary tube has holes in the tube wall through which a wire to be coated is passed.

6. The method according to claim 1 wherein the molten metal is contained in a vessel which has a cylindrical passage for the wire vertically protruded from the bottom of the vessel through which the wire runs without contacting the molten metal in the vessel and the capillary device is a double-walled cap put on said cylindrical passage, the open lower edges of which are immersed in the molten metal so that the molten metal fills the gap formed by the double walls by virtue of a capillary attraction, and the double walls have holes which are provided in a top position of the cap corresponding to that of the protruded cylindrical passage so that the wire runs through the passage and the holes.

7. The method according to claim 1 wherein the distance of the capillary gap is a maximum of 2 mm.

8. The method according to claim 1 wherein the capillary space provided by the small capillary device has a bulged middle part, the width thereof being a maximum of 5 mm. 

2. The method according to claim 1 wherein the small capillary device comprises a pair of walls facing each other and a bottom connecting ends of the two walls.
 3. The method according to claim 1 wherein the small capillary device is a short piece of a capillary tube.
 4. The method according to claim 2 wherein the small capillary device has holes in the walls through which a wire to be coated is passed.
 5. The method according to claim 3 wherein the capillary tube has holes in the tube wall through which a wire to be coated is passed.
 6. The method according to claim 1 wherein the molten metal is contained in a vessel which has a cylindrical passage for the wire vertically protruded from the bottom of the vessel through which the wire runs without contacting the molten metal in the vessel and the capillary device is a double-walled cap put on said cylindrical passage, the open lower edges of which are immersed in the molten metal so that the molten metal fills the gap formed by the double walls by virtue of a capillary attraction, and the double walls have holes which are provided in a top position of the cap corresponding to that of the protruded cylindrical passage so that the wire runs through the passage and the holes.
 7. The method according to claim 1 wherein the distance of the capillary gap is a maximum of 2 mm.
 8. The method according to claim 1 wherein the capillary space provided by the small capillary device has a bulged middle part, the width thereof being a maximum of 5 mm. 